Washington has evidence of Russia firing missiles at Ukraine, White House spokesman Josh Earnest has said.

"We have seen evidence to indicate that the Russians have fired weapons from the Russian side of the border aimed squarely at Ukrainian forces," he said at a press briefing on Tuesday.

"It only underscores the importance of the international community taking action to impose further economic costs on the Russians to get them to reevaluate their strategy in Ukraine," he added.